New England Distribution

Non-native: introduced
(intentionally or
unintentionally); has become naturalized.

County documented: documented
to exist in the county by
evidence (herbarium specimen, photograph). Also covers
those considered historical (not seen in 20 years).

State documented: never been
documented from the
county, but known from the state. May be present. Or,
may be restricted to a small area or a habitat (alpine,
marsh, etc.), so unlikely found in some
counties.

Note: when native and non-native
populations both exist in a county, only native status
is shown on the map.

North America Distribution

Facts About

One of the most invasive weeds in the world, Japanese knotweed is native to Asia, where it is regarded as having medicinal value. It was introduced to North America in the 1870s as an ornamental and forage plant. It is difficult to exaggerate how aggressive this species can be: it has been observed growing through two inches of concrete, and it will regenerate from as little as 5g of stem or root tissue. Control of Japanese knotweed is laborious and expensive.

6×7.
Fallopia japonica
×
Fallopia sachalinensis
→Fallopia×‌bohemica
(Chrtek & Chrtková) J.P. Bailey is an uncommon knotweed hybrid known from
CT, MA, ME,
RI,
VT. It is becoming increasingly more frequent on the landscape. Its leaf morphology is intermediate between
F. japonica and
F. sachalinensis (i.e., they tend to be large and are truncate to cordate at the base). The best discriminating character is the minute, but firm, stoutly conical, 1- or 2-celled hairs found on the abaxial leaf surface (this type of hair not found in the parental taxa; view at 20
× or higher) [Fig. 788, M].